Different overlapping techniques by welding sheets of thermoplastic material are well known. The known techniques can be classified into two large groups: dielectric welding (also called high-frequency welding) and heat welding. Overlapping by dielectric welding is based on applying a high frequency alternating voltage (of the order of 10 MHz) between two plates (electrodes) and between which the sheets to be welded are located. Said voltage causes the sheets to vibrate at molecular level with the subsequent temperature increase for melting said sheets until reaching the melting temperature in the material of the contact area. Then, usually by additionally applying compression to the sheets and after cooling them, overlapping by welding normally takes place.
In contrast, in overlapping by heat welding, said heating is through heat conduction heat (e.g. by means of contact with a hot surface or blowing hot air) or heat radiation (e.g. by infrared) transfer means. To that end, one of the outer surfaces of the sheets or both outer surfaces at the same time to be welded are heated by heat transfer from a heat source. The heat transfer occurs during the time needed to reach the melting temperature in the material of the contact surfaces of the sheets. Then, usually by additionally applying compression between the sheets and similar to the prior art, the overlap is formed by welding after cooling them.
The technique most commonly used to date for forming overlaps by welding sheets of thermoplastic material is high-frequency welding. This is because of its considerable advantage over the other technique in terms of speed. The reason is that dielectric heating takes place from the contact area of the sheets outwards, so the melting of the material of the contact area takes place very quickly.
On the other hand, a drawback of the dielectric heating technique is that it cannot be suitably applied to any type of thermoplastic materials. The characteristic of thermoplastic materials indicating their ease in being welded with dielectric heating is the so-called dielectric loss factor. Thermoplastics with a high dielectric loss factor, such as PVC or PU for example, are easily weldable with dielectric heating. However, thermoplastics such as PTFE (Teflon®), which have a low dielectric loss factor, cannot be suitably welded with dielectric heating. PTFE is commonly used as an electrical insulation material so the heat welding technique can be useful for welding sheets of PTFE having said use.
Generally, both conventional overlap dielectric welding and heat welding techniques present the following technical problem.
When the overlap by heat welding technique is applied, if the heat source acts on a single outer surface, taking into account that the melting temperature must be reached in the contact area, to that end the temperature in the outer surface must be significantly higher, this excess temperature can lead to overheating the material of the outer surface and therefore to degradation thereof. Furthermore, this case further has the added problem that the welding process consumes a relatively excessive amount of time as the heat source acts on a single outer surface. If in contrast the heat source is applied to the two outer surfaces, the temperature of said surfaces will also be increased to reach the melting temperature in the contact area and subsequently the technical and aesthetic characteristics or properties of the outer surfaces will be able to be altered.
An example of alteration (without reaching degradation) of the technical and aesthetic characteristics and properties of the outer surfaces of sheets overlapped by welding can be seen in roller shades. As mentioned, such shades are characterized in that they incorporate a hem and/or joint to which an overlap is applied by welding. In this particular case, the application of the conventional dielectric welding technique usually causes deterioration of the outer surfaces of the shade in the area of the overlap. This deterioration becomes apparent in that the shade in said area is no longer translucent, changing in color, tone, roughness and gloss such that it debases the visual and aesthetic aspect of the shade as a whole.
Generally, temperature can alter technical and aesthetic characteristics and properties of the outer surfaces and of the overlap; such technical properties include mechanical properties (e.g. tensile strength of the overlap, hardness or roughness of the surfaces), physical properties (e.g. electrical conductivity), chemical properties (e.g. water resistance), etc. and aesthetic properties including color, tone, roughness, gloss, etc. The present invention attempts to provide a method and apparatus for forming overlaps by welding sheets such that the technical and aesthetic characteristics or properties thereof do not significantly vary on at least one of the outer surfaces, being particularly useful for manufacturing roller shades, awnings, parasols, canvases for covering facades, etc.